1. Field of the Invention
The present invention relates to a phase control method and apparatus, and particularly to a phase control method and apparatus for eliminating a phase difference between two signals which originate from a single source, but have been carried over two different transmission lines.
2. Description of the Related Art
To accommodate a variety of network interfaces having different specifications, Synchronous Digital Hierarchy (SDH) transmission systems are organized, in general, by a number of network devices with increasingly high system complexity. Further, SDH systems often employ dual-link optical transmission media for higher reliability and availability of service; that is, a single-sourced digital signal is carried over two independent transmission lines. When one transmission line is accidentally disrupted for any reason or intentionally disabled for maintenance operations, the ongoing signal transmission is automatically switched to the other line without any momentary dropout of the transmission signal. For this purpose, the physical layer of such SDH transmission systems is equipped with a non-interruptive switchover mechanism.
FIG. 13 outlines a conventional transmission system having a non-interruptive switchover capability. In this simplified block diagram, a digital transmission signal is delivered from a sending station 200 to a receiving station 300 via a dual transmission link. More specifically, a transmission unit 201 in the sending station 200 produces a path signal P having a Network Node Interface (NNI) frame format. This path signal P is transported over two independent transmission lines A and B in a parallel fashion. Since the lengths of these two lines A and B are different in general, the two path signals P will arrive at the same receiving station 300 at different times. This fact leads to a need for the phase synchronization of signal frames at the receiving end to enable non-interruptive switchover operations.
To solve the problem of such a timing difference, the receiving station 300 employs two delay memory units m1 and m2, which give a certain amount of delay time to one of the two path signals that arrives earlier. With an appropriate setup, the output signals of these delay memory units m1 and m2 will be in phase. Now that two in-phase path signals P are available, the receiving station 300 can select either signal by using a selector 301 in a non-interruptive manner.
FIG. 14 shows a frame structure of path signals, which is simplified for easier understanding of the concept. The SDH standards define a Path Overhead (POH) as part of each NNI frame, which consists of nine bytes for transmission control purposes. Among those control bytes in a POH, J1 byte (path trace byte) and/or H4 byte (multiframe indicator byte) is used to carry an appropriate identifier ID. This identifier ID is inserted not to every frame, but only to the first frame of every predefined multiframe. The length of this multiframe signal is determined taking into consideration the distance difference between the two transmission lines A and B. The transmission device 201 assembles such multiframe signals so that the phase matching will be accomplished, even in such a case where the distance difference causes a time difference exceeding one NNI frame interval. Actually, the arrangement of identifiers ID shown in FIG. 14 will allow the receiving station 300 to compensate for any phase differences if they are within one multiframe length. In this way, the conventional SDH transmission system performs phase adjustment operations by detecting, on the receiving end, a difference in the arrival times of predefined identifiers ID that are embedded in multiframe path signals.
The above-described conventional transmission systems, however, are unable to interwork with other existing network devices having no capabilities of inserting identifiers ID into multiframe signals. This is because the non-interruptive switchover mechanism implemented in the conventional systems requires the sending end to assemble multiframe signals in a prescribed way, as does the sending station 200 in the system of FIG. 13.
Another drawback of the conventional systems is that they consumes control bytes in POH to insert identifiers ID for the phase adjustment purpose, although some applications may require such control bytes for other usage.
Further, the conventional phase detection scheme requires identifiers ID to be inserted into multiframe signals at sending stations, meaning that the comparison range (i.e., a time range in which two path signals should be monitored to detect their phases) is determined not at each receiving end, but on the side of sending stations. This leads to inefficiency in the phase control.